Effect of Y/Zn ratio on microstructure and properties of as-extruded Mg-Y-Zn alloys

Abstract

In this work, four Mg-Y-Zn alloys with specific Y/Zn (wt%) ratio have been prepared. The effects of Y/Zn (wt%) on the microstructure of extruded Mg-Y-Zn alloy were investigated by SEM and TEM. The mechanical properties and corrosion behavior of extruded Mg-Y-Zn alloy were also studied by tensile test, electrochemical and immersion measurements. The corrosion mechanism was studied by SEM observation of the surface morphology of the samples after immersion corrosion. The result indicates that the as-extruded Mg-6Y-3Zn alloy consists of α-Mg, W phase and long-period stacking ordered structure (LPSO) phases. Two types of LPSO structure (18 R and 14 H) appear in the asextruded alloys. The volume fraction of LSPO increases with the increase of Y and Zn atoms witht the same Y/Zn mass ratio. Mg-9Y-3Zn alloy has the best comprehensive mechanical properties. Its yield strength, ultimate tensile strength and elongation are 230 MPa, 327mpa and 23% respectively, because the volume fraction of LSPO phase in the alloy is the highest. In addition, the higher volume fraction of LPSO is beneficial to enhance the corrosion resistance of Mg-9Y-3Zn alloy.Magnesium alloys have a series of performance advantages, such as low density, good specific strength and stiffness, good casting performance and processing performance, and also has low elastic modulus, vibration and noise reduction, etc, which have been widely applied in 3 C, aerospace and automobile fields [1-3]. However, the unavoidable problem is the low strength and poor corrosion resistance of magnesium alloys [4][5][6]. Therefore, researchers have done a lot of research on the strengthening and corrosion behavior of magnesium alloys [7][8][9][10].In improving the strength of magnesium alloys, magnesium rare earth alloys are the most successful systems. Recent studies have found that adding a small amount of Zn to certain Mg-RE alloys can generate a novel longperiod stacking ordered structure (referred to as long-period structure, LPSO structure) under appropriate addition amounts and process conditions. In-depth research shows that there are three types of ternary equilibrium phases in Mg-Zn-Y alloys, which are Mg 3 YZn 6 (I-phase icosahedral quasi-crystal structure and quasi-periodic order), Mg 3 Y 2 Zn 3 (W-phase face-centered cubic structure) and Mg 12 ZnY (X-phase long period stacked ordered structure, including 6 H, 14 H and 18 R) [8]. At the same time, it is generally believed that the phase composition of the alloy depends on the Y/Zn molar ratio [9]. It is reported that LSPO is an important strengthening phase in Mg-Y-Zn alloy, and the common structures are 18 R and 14 H [10]. After heat treatment at 350°C-500°C, 18 R structure will be transformed into 14 H LPSO structure [11]. Wang J F et al reported that the LPSO structural phase can improve the mechanical properties of magnesium alloys while maintaining the good damping capacity of the alloys, and the LPSO phase arranged along the extrusion direction can act as the hardening phase [12]. It is known that hot extrusi...

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